CAPE CANAVERAL, Fla. (AP) — Fuel leaks and a possible crack discovered during final liftoff preparations threatened to postpone the launch of NASA’s mighty new moon rocket Monday morning on its shakedown flight with three test dummies aboard.

As precious minutes ticked away, NASA repeatedly stopped and started the fueling of the Space Launch System rocket with nearly 1 million gallons of super-cold hydrogen and oxygen because of a leak.

The fueling already was running nearly an hour late because of thunderstorms off Florida’s Kennedy Space Center

The leak of highly explosive hydrogen appeared in the same place that saw seepage during a dress rehearsal back in the spring.

Then a second apparent hydrogen leak turned up in a valve that had caused trouble in June but that NASA thought it had fixed, officials said.

Later in the morning, a crack or some other defect was spotted on the core stage — the big orange fuel tank with four main engines on it — with frost appearing around the suspect area, NASA officials said.

The launch represents a milestone in America's quest to put astronauts back on the lunar surface for the first time since the Apollo program ended 50 years ago.

The 322-foot (98-meter) rocket is the most powerful ever built by NASA, out-muscling even the Saturn V that took the Apollo astronauts to the moon.

No astronauts were inside the rocket's Orion capsule. Instead, the test dummies, fitted with sensors to measure vibration, cosmic radiation and other conditions, were strapped in for the six-week mission, scheduled to end with the capsule's splashdown in the Pacific in October.